<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>Document</title>
</head>
<body>
  <script>
    // 字符串不可变
    var str = 'qwerw'
    console.log( str.indexOf('f') )  // -1
    console.log( str.indexOf('f') )  // -1
    console.log( str.indexOf('w', 2) )  // 4

    var arr = ['q', 'w', 'e', 'r', 'w']
    console.log( arr.indexOf('w', 2 ) )  // 4

  </script>
</body>
</html>